1. Field of the Invention
The present invention relates to an NOx-concentration detecting apparatus for detecting the concentration of nitrogen oxides, or harmful emissions, contained in exhaust gases from various combustion apparatus, including internal combustion engines.
2. Description of the Related Art
NOx-concentration detecting apparatus using an NOx sensor for detecting the concentration of nitrogen oxides (NOx) contained in exhaust gases from internal combustion engines and the like are disclosed, for example, in European Patent Application Laid-Open No. 0678740A1 and SAE Paper No. 960334, pp. 137-142, 1996. An NOx sensor used in such a conventional NOx-concentration detecting apparatus is composed of oxygen-ion conductive solid electrolyte layers that form a first measurement space and a second measurement space. The first measurement space communicates with the gas to be measured (hereinafter called "a measurement gas") via a first diffusion-controlling layer, and the second measurement space communicates with the first measurement space via a second diffusion-controlling layer. Furthermore, the solid electrolyte layer of the first measurement space is sandwiched between porous electrodes so as to form a first oxygen-pumping cell and an oxygen-concentration-measuring cell. Also, the solid electrolyte layer of the second measurement space is sandwiched between porous electrodes so as to form a second oxygen-pumping cell.
In the thus-configured NOx-concentration detecting apparatus, the first oxygen-pumping cell is energized so that an output voltage from the oxygen-concentration-measuring cell attains a predetermined value, thereby pumping out oxygen from the first measurement space and thus controlling the concentration of oxygen contained in the first measurement space at a constant level. At the same time, a constant voltage is applied to the second oxygen-pumping cell to thereby pump out oxygen from the second measurement space. As a result, the NOx concentration of a measurement gas can be obtained from current flowing through the second oxygen-pumping cell.
A measurement gas, e.g., exhaust from an internal combustion engine or the like, contains gas components other than NOx, such as oxygen, carbon monoxide and carbon dioxide. Thus, in the aforementioned NOx-concentration detecting apparatus, first, current is applied to the first oxygen-pumping cell to thereby pump out most of the oxygen from a measurement gas contained in the first measurement space. Then, in the second measurement space into which the oxygen-removed measurement gas flows, a constant voltage is applied to the second oxygen-pumping cell in a direction such that oxygen is pumped out from the second measurement space. As a result, NOx contained in the measurement gas is decomposed into nitrogen and oxygen by means of the catalyzing function of the porous electrodes of the second oxygen-pumping cell, and the thus-generated oxygen is then pumped out from the second measurement space. Thus, the NOx concentration of the measurement gas can be obtained from current flowing through the second oxygen-pumping cell without being influenced by other gas components contained in the measurement gas.
In such an NOx-concentration detecting apparatus, the concentration of nitrogen oxides is obtained based on current flowing through the second oxygen-pumping cell. The current flow is usually on the HA scale, and is considerably smaller than the current flowing through the first oxygen-pumping cell. When the current flowing through the second oxygen-pumping cell varies by 1 .mu.A, the concentration of nitrogen oxides varies by 100 to 200 ppm. As a result, sensor operation is highly sensitive to operating conditions, so that the NOx sensor may become unusable under certain conditions. Also, when the concentration of NOx is measured at a high resolution of, for example, 1 ppm, the above microscopic variations in the concentration of nitrogen oxides present a significant measurement problem.
When the NOx sensor becomes unusable due to a failure or the like, knowledge of the operating conditions under which the NOx sensor had been used prior to that time is important in investigating the cause of the failure. If the cause can be identified, countermeasures can be taken so as to effectively improve the NOx sensor.
However, conventionally, when there was a need to know the operating conditions under which the failed NOx sensor was used, there was no choice but to inquire of an operator about his/her memory of the operating conditions. Thus, a problem with conventional NOx sensors is that it is difficult to make effective improvements.
Also, the concentration of NOx in the measurement gas is determined based on characteristics correlated to the current flowing through the first-oxygen pumping cell, the current flowing through the second oxygen-pumping cell, and the concentration of NOx in the measurement gas. However, these characteristics differ somewhat among NOx sensors. Accordingly, when the same correlations are used among a number of NOx sensors for measuring NOx concentration, a high degree of measurement accuracy may not be obtained.